Participants in this study will be randomly assigned to receive either two different doses of CY or placebo (an inactive pill) for 6 months. Participants who receive placebo during the 6 months will have the option of receiving CY for an additional 6 months. Participants will undergo magnetic resonance imaging (MRI) scans of the brain. Measures of cerebral spinal fluid biomarkers or neurodegeneration, neuroinflammation, and neuroimmune activation will be taken. In addition, peripheral lymphocyte subsets and peripheral markers of inflammation will be assessed.